Adaptive predistortion for a transmit system with gain, phase and delay adjustments

ABSTRACT

Systems and methods relating to the provision of gain, phase and delay adjustments to signals to be used by a predistortion subsystem. A portion of an input signal is delayed by delay elements prior to being received by the predistortion subsystem. The delayed input signal portion is also received by a feedback signal processing subsystem that adjusts the gain and phase of the feedback signal based on the delayed input signal portion. The adjusted feedback signal is used, along with the delayed portion of the input signal, to determine an appropriate predistortion modification to be applied to the input signal.

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/613,856 filed Jul. 3, 2003.

FIELD OF THE INVENTION

The present invention relates generally to power amplification systemsand is specifically applicable but not limited to power amplificationsystems using a Chireix architecture.

BACKGROUND TO THE INVENTION

The recent revolution in communications has caused a renewed focus onwireless technology based products. Mobile telephones, handheldcomputers, and other devices now seamlessly communicate using wirelesstechnology. One component that forms the core of such technology is theamplifier. Wireless devices require high efficiency amplifiers to notonly extend the range of their coverage but also to conserve the limitedbattery power that such devices carry.

One possible architecture which may be used for such a power amplifieris called a Chireix architecture. Named after Henry Chireix who firstproposed such an architecture in the 1930s, the Chireix architecture hasfallen out of favor due to its seemingly inherent limitations. However,it has recently been revisited as it provides some advantages that otherarchitectures do not have.

While the Chireix architecture provides some advantages, the processwhich the input signal undergoes also introduces some drawbacks.Specifically, distortions are introduced into the signal by thecomponents in the Chireix based amplifier/modulator system. Thesedistortions may also change over time and may therefore lead to atime-varying “drift” or change in the signal. Such distortions,time-varying or not, have led to problems that are not only inconvenientbut expensive as well.

Based on the above, there is therefore a need for an amplifier systemwhich provides the benefits of a Chireix based amplifier but which alsocompensates for or avoids the distortions which a Chireix basedamplifier introduces. Such an amplifier system should adjust todiffering conditions, preferably with little or no user intervention. Itis therefore an object of the present invention to provide alternativeswhich mitigate if not overcome the disadvantages of the prior art.

SUMMARY OF THE INVENTION

The present invention provides systems and methods relating to theprovision of gain, phase and delay adjustments to signals to be used bya predistortion subsystem. A portion of an input signal is delayed bydelay elements prior to being received by the predistortion subsystem.The delayed input signal portion is also received by a feedback signalprocessing subsystem that adjusts the gain and phase of the feedbacksignal based on the delayed input signal portion. The adjusted feedbacksignal is used, along with the delayed portion of the input signal, todetermine an appropriate predistortion modification to be applied to theinput signal.

In a first aspect, the present invention provides a system forprocessing an input signal, the system comprising:

an adaptive predistortion subsystem for receiving said input signal andfor producing a predistorted signal by applying a deliberatepredistortion to said input signal;

a signal processing subsystem receiving and processing said predistortedsignal and producing a system output signal;

a feedback signal processing subsystem for receiving and processing afeedback signal derived from said system output signal; and

a delay subsystem for providing a delay to a replica of said inputsignal to produce a delayed signal, said delayed signal being used bysaid adaptive predistortion subsystem and said feedback processingsubsystem, wherein

said adaptive predistortion subsystem distorts said input signal tocompensate for distortions in said system output signal;

said signal processing subsystem decomposes said predistorted signalinto separate components, each of said separate components beingprocessed separately;

said processing subsystem combines said components after processing toproduce said system output signal;

an output of said feedback processing subsystem being used by saidadaptive predistortion subsystem;

said deliberate predistortion applied to said input signal by saidadaptive predistortion subsystem to produce said predistorted signal isadjusted based on said system output signal.

In a second aspect the present invention provides a method of processingan input signal to produce a system output signal, the methodcomprising:

a) receiving said input signal;

b) applying a deliberate predistortion to said input signal to result ina predistorted signal, said deliberate predistortion being based oncharacteristics of said system output signal;

c) decomposing said predistorted signal into at least two componentsignals;

d) combining said at least two component signals to produce said systemoutput signal;

e) adjusting said deliberate predistortion based on said system outputsignal

f) concurrent with steps b)-e), executing the following steps:

-   -   f1) delaying a replica of said input signal;    -   f2) diverting a diverted replica of said system output signal to        a feedback path;    -   f3) processing said diverted replica to produce a corrected        version of said diverted replica; and    -   f4) determining said deliberate predistortion using said        corrected version and said replica of said input signal.

In a third aspect the present invention provides an adaptivepredistortion subsystem for use with a signal processing system whichproduces a system output signal, the predistortion subsystem comprising:

determining means for determining a deliberate predistortion to beapplied to an input signal;

adjustment means for applying said deliberate predistortion to saidinput signal;

update means for periodically updating said determining means based onsaid system output signal wherein said adaptive predistortion subsystemuses

an output of a delay subsystem for delaying said input signal and

an output of a feedback processing subsystem for processing a feedbacksignal derived from said system output signal to determine saiddeliberate predistortion.

In a fourth aspect the present invention provides a method ofinitializing a phase correction to be applied to a feedback signal, saidfeedback signal to be used in determining a deliberate predistortion fora signal processing system, the method comprising:

a) initiating a coarse delay search

b) selecting a time window of W samples of said feedback signal and aninput signal with a predetermined sample delay increments of δ betweensamples

c) calculating an inner product P_(δ) by performing a complex multiplyand accumulate process for the W samples in the time window

d) storing a maximum |P_(δ)| found

e) repeating steps c) and d) for subsequent time windows andincrementing δ by a predetermined amount for each time window

f) repeating steps b)-e) for a fine delay search using fractional sampleincrements to cover a predetermined delay range, said delay range beingcentered on a maximum delay increment δ_(max) found during said coarsedelay search.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be obtained by consideringthe detailed description below, with reference to the following drawingsin which:

FIG. 1 is a block diagram of a Chireix architecture amplifier subsystem;

FIG. 2 is a block diagram of an amplifier system using the subsystem ofFIG. 1 and a predistortion subsystem;

FIG. 3 is a detailed block diagram of the internal components of thepredistortion subsystem;

FIG. 4 illustrates the amplifier system of FIG. 2 and incorporating afeedback signal used by the predistortion subsystem of FIG. 3;

FIG. 5 is a block diagram of a delay line circuit which may be used inthe system of FIG. 4;

FIG. 6 is a circuit which may be used to implement the magnitudeadjustment explained below;

FIG. 7 is a detailed block diagram of one embodiment of a Chireixamplifier subsystem;

FIG. 8 is a block diagram of a signal processing system according toanother embodiment of the invention; and

FIG. 9 is a detailed block diagram of a system incorporating the featureshown in FIGS. 3, 4 and 8.

DETAILED DESCRIPTION

For clarity, the following terms are to be used with the followingdefinitions:

AM (amplitude modulation) refers to the AM of an RF (radio frequency)signal and is equal to the magnitude of the RF signal's complex baseband equivalent

PM (phase modulation) refers to the PM of an RF signal and is equal tothe phase of the RF signal's complex base band equivalent

Referring to FIG. 1, a block diagram of a Chireix architecture amplifiersubsystem 10 is illustrated. A signal decomposer 20 receives an inputcomplex baseband signal 30. Phase modulated signals 80A, 80B areproduced after the decomposed output of the decomposer 20 are phasemodulated by phase modulation circuitry 85A, 85B. These phase modulatedsignals 80A, 80B are received by power amplifiers 90A, 90B. The phasemodulated signals are thus amplified by the power amplifiers 90A, 90Band are received by a signal combiner 100. The system output signal 110(an RF signal corresponding to the input baseband signal 30) is outputfrom the combiner 100 and is an amplified and modulated version of theinput signal 30. Phase modulation of the phase modulated signals 80A,80B is executed by the signal decomposer 20. The input signal 30 isseparated into at least two components and these at least twocomponents, after phase modulation, are the signals 80A, 80B.

As noted above, the Chireix architecture amplifier subsystem 10 has beenknown to introduce distortions in the system output signal 110. Tocompensate for such distortions, a predistortion subsystem 120 isprovided. Referring to FIG. 2, the predistortion subsystem 120 receivesthe input signal 30 and produces a predistorted signal 130. Thepredistorted signal 130 is received by the amplifier subsystem 10. Theamplifier subsystem then produces the system output signal 110.

The distortions for which the predistortion subsystem is to compensatemay come as a phase distortion, a magnitude distortion, or as acombination of both. It has been found that, without predistortion, thesystem output signal 110 has an amplitude modulation (AM) that is notequal to the expected and desired AM. Furthermore, the phase modulation(PM) of the system output signal 110, if predistortion is not present,deviates from the expected and desired PM. Experiments have found thatthe AM distortion or error (magnitude distortion) depends on the AM ofthe input signal. Also, it has been found that the PM distortion (orphase distortion) depends on the AM of the input signal.

As noted above, one solution to the above issues is to predistort theinput signal as detailed in FIG. 2. Further details on this approach canbe found in co-pending application entitled Predistortion Circuit For aChireix Power Amplifier Transmit System and filed with the US Patent andTrademark Office, the whole of which is incorporated herein byreference. While the predistortion solution does work, it is not asrobust and as fault tolerant as may be desirable. An adaptivepredistortion subsystem 200, as illustrated in FIG. 3, would compensatefor changing conditions and for other distortions which the systemoutput signal may have.

Referring to FIG. 3, a block diagram of such an adaptive predistortionsubsystem is illustrated. The adaptive predistortion subsystem 200 ofFIG. 3 maybe used in place of the predistortion subsystem 120 of FIG. 2.

While an analog implementation of the subsystem 200 is possible, it hasbeen found that a digital implementation was simpler to achieve. Assuch, the following description assumes that the input signal 30 is adigital signal having a digital representation of the desired AM and PMof the desired output RF signal. Digital AM/AM predistortion modifiesthe magnitude of the complex digital input signal such that the RFoutput signal has the desired AM, despite the distortion. Digital AM/PMpredistortion modifies the phase of the complex digital input signalsuch that the RF output has the desired PM, despite the distortion.

As can be seen in FIG. 3, multiple components are involved in theadaptive predistortion subsystem 200: a Cartesian to polar conversionunit 210, a magnitude value lookup table (LUT) block 220, a magnitudeupdate block 230, a magnitude delay block 240, a phase value lookuptable (LUT) block 250, a phase value update block 260, a phase delayblock 270, and an adder 280. The digital input signal 30 is converted bythe conversion unit 210 from Cartesian coordinates to polar coordinates.The magnitude of the converted signal is then received and used by thelookup table blocks 220, 250 to determine the proper amount ofpredistortion to be applied. The phase lookup table 250 adds the phasedistortion to the converted signal by way of the adder 280. Thepredistorted signal is then passed on to the amplifier subsystem 10.

It should be noted that the predistortion modification, defined as anydeliberate distortion which has been introduced to the input signal tochange at least the phase or magnitude of the input signal, can takemany forms. FIG. 3, provided merely for illustrative purposes, depictstwo types of predistortion—phase predistortion and magnitudepredistortion. These two types, separately or together, can make up thepredistortion modification. In some applications, only a magnitude typepredistortion modification may be required while in others only a phasetype predistortion modification is required. In the embodiment explainedhere, the two types of predistortion, together comprise thepredistortion modification.

To account for changing conditions and to acquire appropriate LUTentries, a feedback mechanism is employed to adjust or adapt the lookuptable entries in lookup table blocks 220, 250. Delay blocks 240, 270ensure that the feedback sample is mated with the proper value of theinput signal waveform when processing and updating the lookup tableentries in lookup table blocks 220, 250.

The conversion unit 210, while present, is not necessary but merelyconvenient and makes the predistortion easier to accomplish. As is wellknown, signal representations using Cartesian coordinates take the formof z=x+j y where x and y are the real and imaginary components. Polarcoordinates take the form of z=Ae^(jφ) where the magnitude of the signalis A and its phase is φ. Since both the magnitude and the phase of thesignal is to be modified by the predistortion subsystem, it is clearlymore convenient to accomplish this if the signal were in polarcoordinates. Again as is well known, A=(x²+y²)^(1/2) while φ=tan⁻¹(y/x).Once the signal has been converted into polar coordinates, adjusting themagnitude is as simple as replacing the digital representation of A byanother number. Similarly, the phase can be adjusted by adding a phasecorrection to the phase of the signal.

After the digital input signal is received and converted by theconversion unit 210, the signal is now represented by two values—amagnitude value 290 and a phase value 300. FIG. 3 shows the differentsignal paths followed by these values—one path for the magnitude value290 and a second path for the phase value 300.

As noted above, the magnitude value 290 can be easily replaced by thepredistorted magnitude value. This is done by way of magnitude lookuptable block 220. The lookup table internal to the magnitude lookup tableblock 220 represents an input/output relationship with the input beingthe undistorted magnitude and the output being the predistorted signalmagnitude. Thus, if the magnitude LUT block 220 has a table entry withan input value of 0.5 and an output value of 0.4, then if theundistorted magnitude value received by the magnitude LUT block 220 is0.5, then this value is replaced with 0.4 as the output of the magnitudeLUT block 220. Based on the LUT (lookup table) entries, the magnitude ofthe undistorted signal is therefore replaced with the desiredpredistorted magnitude.

Similar to the above, the phase value of the converted input signal isadjusted as well. As can be seen in FIG. 3, the magnitude value 290 isalso received by the phase lookup table block 250. The phase lookuptable block 250, based on the magnitude value, determines the properamount of phase adjustment and adds this phase adjustment to the phasevalue 300 by way of the adder 280. The phase lookup table block 250 alsohas a lookup table resident within the phase LUT block 250 that detailsthe appropriate phase adjustments for given magnitude values.

While the above described magnitude LUT replaces a desired value for thereceived magnitude, other implementations are possible. Instead of adirect replacement value, the magnitude LUT may provide a correctivevalue to the received magnitude. This corrective value can, depending onthe implementation, be an additive or a multiplicative corrective value.

The adaptive predistortion subsystem 200 in FIG. 3 is adaptive in thatthe values of the lookup table entries in the lookup table (LUT) blocks220, 250 change over time to account for changing conditions or foracquiring appropriate LUT entries. This adaptability is implemented byway of a feedback signal tapped from the system output signal 110.Referring to FIGS. 3 and 4, two feedback signals, a magnitude feedbacksignal 310 and a phase feedback signal 320, are received by themagnitude value update block 230 and by the phase value update block 260respectively. These two feedback signals result from processing of thesystem output signal 110 by the analog/digital (A/D) converter 330, theCartesian to polar conversion unit 340, demodulation module-335, andfiltering module 337. As can be seen in FIG. 4, the system output signal(an analog signal) is tapped and this tapped signal 345 is received bythe A/D converter 330 for conversion from an analog to a digital signal.

After conversion to a digital signal, the feedback signal is convertedfrom Cartesian to polar coordinates by the conversion unit 340. Thetapped signal 345 is thus represented by the two feedback signals—themagnitude feedback signal 310 and the phase feedback signal 320. Asmentioned above, both these feedback signals are received by theirrespective update blocks 230, 260.

Once the two digital feedback signals are received, they are thencompared with the delayed input signal coming from the delay blocks 240,270. The updated values for the LUT entries are then calculated andentered into their respective lookup tables. It should be noted that thecomparison may be done by subtracting the feedback signals from thedelayed input signal.

To further elaborate on the above process, the update process isdependent on the difference between the tapped system output signal 345and the input signal 30. This difference is, of course, taken after bothsignals are in polar coordinates. The magnitude and phase errors aredefined as:e _(m)(k)=|z(k)|−x(k)|e _(φ)(k)=(∠z(k)−∠x(k))where

-   -   e_(m)(k)=magnitude error    -   e_(φ)(k)=phase error    -   z(k)=magnitude of feedback signal (signal 310 )    -   x(k)=magnitude of input signal (signal 290)    -   φz(k)=phase angle of feedback signal (signal 320)    -   ∠x(k)=phase angle of input signal (signal 300)

For the magnitude LUT entries in the magnitude LUT block 220, twovariables are defined and used in the process:δ_(F)=−μ_(F) ·e _(m)(k)where

-   -   δ_(F)=update quantity dependent on the differences between the        magnitudes of the input signal and the feedback signal    -   μ_(F)=an update speed parameter μ (user selectable), typically        μ_(F)>0.

Since the magnitude LUT has LUT entries, each entry is given an entryaddress of n with 0≦n≦−1, N being the maximum number of entries in theinternal magnitude LUT in the magnitude LUT block 220.

An interpolation distance s is defined as s=αM−n where n=└αM┘ (or thelargest integer value less than or equal to αM), M=|x(k)|, and α is ascaling value applied such that the magnitude range (e.g. 0≦M<1) ismapped to a table index range 0≦n≦(−1).

The table entries are thus updated using the following formulae (n beingone table address and n+1 being another table address):F _(n)(k+1)=F _(n)(k)+(1−s)·δ_(F) iff 0≦n≦(−1)F _(n+1)(k+1)=F _(n+1)(k)+(s)·δ_(F) iff 0≦n+1≦(−1)where

-   -   F_(n)(k)=table entry n for time sample k    -   F_(n)(k+1)=table entry n for time sample k+1    -   F_(n+1)(k)=table entry for n+1 for time sample k    -   F_(n+1)(k+1)=table entry n_(—)1 for time sample k+1

From the above equations, it should be clear that one, two or no entriesin the internal magnitude LUT are updated depending on the value of αM.Thus, for this implementation, depending on the value of αM, one ofF_(n) and F_(n+1) is updated, both are updated, or neither is updated.Other implementations, depending on the circumstances and needs of theparticular application, may update other numbers of entries.

For the phase LUT entires, an analogous process is used in the phaseupdate block 260. An update quantity is defined:δ_(G)=μ_(G) ·e _(φ)(k)where

-   -   δ_(G)=update quantity dependent on the differences between the        phase angles of the input signal and of the system output signal    -   μ_(G)=an update speed parameter (user selectable) where        0≦μ_(G)<1

Using the parameter s as defined above for the magnitude LUT, the phaseLUT entires can be updated using the following formulas:G _(n)(k+1)=G _(n)(k)+(1−s)·δ_(G) iff 1≦n≦(−1)G _(n+1)(k+1)=G _(n+1)(k+1)+(s)·δ_(G) iff 1≦n≦N

Again, much like the magnitude LUT entry update process, the updateprocess for the phase entries will, depending on the value of αM, updateone, two, or none of the phase LUT entries.

It should be noted that LUT adaptation may involve updating more thantwo entries with some weighting applied. As an example, the weightingmay depend upon the distance of the LUT entry from the update point. Thediscussion herein is based upon the use of two entries and the use of aninterpretation distance.

As an example of the above processes, a phase entry update will beillustrated. For this example, the following values are assumed:N=6M≦1α=5x(k)=0.35 exp(j·2·7)z(k)=0.2 exp(j·3·1)μ_(G)=0.1

Thus, M=0.35 and αM=1.75. Thus, n=1 (since the lowest integer value lessthan or equal to 1.75=1) and n+1=2. From these values, s=1.75−1=0.75.Given that z(k)=0.2 exp(j·2.3), the e_(φ)(k)=−0.4. Thus,δ_(G)=−(0.1)(−0.4)=+0.04. The required correction for G₁ is therefore(1−s)·δ_(G)=(1−0.75)(+0.04)=0.25·(+0.04)=0.01. For G₂, the correction iss·δ_(G)=(0.75)(0.04)=0.03. The new values are therefore:G ₁(k+1)=G ₁(k)+0.01G ₂(k+1)=G ₂(k)+0.03

This update is illustrated by the values in the following table: LUTcontent LUT content Address (n) before update Correction after update 0−1 0 −1 1 2 0.01 2.01 2 −0.5 0.03 −0.47 3 −0.5 0 −0.5 4 0.5 0 0.5 5 2 02

It should be noted that the above process also takes into account thelookup of values that are not to be found in the internal lookup tables.Linear interpolation using s=αM−n is used for magnitude values thatcannot be found in the lookup table entries. As an example, if themagnitude is given as 0.35 but the only table entries are 0.3 and 0.4,linear interpolation is used. The following formula is used to findvalues not found in the lookup tables:F(M)=(1−s)·F _(n) +s−F _(n+1)where

-   -   F_(n)=table value A    -   F_(n+1)=table value B with the desired value being between table        values A and B    -   n=└αM┘ (the largest integer value less than or equal to αM)    -   s=αM−n    -   α=scaling value as defined above.

From the above discussion, it should be fairly clear that two of thevalues. underpinning most of the calculations are the values fore_(m)(k) and e_(φ)(k), the magnitude and phase errors. While theequations above for these two values will provide acceptable answers,better results may be had by taking into account the phase, magnitude,and delay effects of the feedback signal (i.e. the tapped system outputsignal 345). However, to be able to do this, the input signal must beproperly delayed so that samples from the interpolated input waveform,as obtained from the delayed input signal samples, are mated with therelevant system output signal sample. Such proper delaying shouldtherefore take into account most, if not all, the time delay involved inthe processing production, and feedback of the system output signal(round trip delay). This round trip delay is denoted as τ (seconds) and,before the LUT updating begins, the delay blocks 240, 270 acquires thedelay and delays the input signal accordingly so as to time-align theinput signal samples with the incoming feedback signal.

To implement this delay, a delay line is used with a depth of K, meaningK samples of the input signals may be stored in the delay line. Asshould be clear, each of the K samples were sampled at differentinstances in time. The value of K is predetermined and should be enoughto allow for the maximum possible path delay between the input signaland the feedback signal. These delays are due to a combination of any ofthe following: digital pipelining, analog and digital filter groupdelays, analog propagation delays, and the system and implementationdependent delays.

Because of this delay, a time delayed version of the input signal,x_(δ)(k) is defined and this is ideallyx _(δ)(k)=x(k−δ)where

-   -   δ=τ·F_(s)    -   F_(s)=signal sampling rate    -   τ=delay (normal trip delay between input and system output        signal feedback)

To obtain a better result for x_(δ)(k), linear interpolation is used toallow for fractional values of δ. Thus, the delay is divided into twoparts κ, the integer part of the sample (representing a discrete sampledelay at the sample rate F_(s)), and ν, a fractional sample of thedelay.

Using this notation, the delayed portion of the input signal can berepresented as:x _(δ)(k)=(1−ν)·x(k−κ)+ν·x(k−κ+1)where

-   -   δ=τ·F_(s)    -   κ=└δ┘    -   ν=δ−κ        As can be seen, for an integer δ, x_(δ)(k)=x(k−δ).

The above Cartesian equation can be applied to polar representations byhaving separate delay lines for magnitude (|x_(δ)(k)|) and phase(∠x_(δ)(k)) using the sequences |x(k)| and ∠x(k). These are given by:∠x _(δ)(k)=(1−ν)·∠x(k−κ)+ν·∠x(k−κ+1)|x _(δ)(k)|=(1−ν)·|x(k−κ)|+ν·|x(k−κ+1)|It should be fairly clear that x_(δ)(k) is calculated from the samplesx(k), x(k−1), x(k−2), . . . x(k−κ), samples of the input signal taken attime k, k−1, k−2, . . . k−κ.

The above equations for ∠x_(δ)(k) has a peculiarity that is due to theway angle values work. Since ∠x(k−κ) and ∠x(k−κ+1) are represented bymodulo 2π radians (360 degrees) and since −π≦∠x(k)≦π, then errors couldeasily occur.

Thus, if −π≦∠x(k)≦π, and if |∠x(k−κ+1)−∠x(k−κ)|≧π, then∠x _(δ)(k)=(1−ν)·∠x(k−κ)+ν·(∠x(k−κ+1)+2π) if ∠x(k−κ+1)≦∠x(k−κ)∠x _(δ)(k)=(1−ν)·∠x(k−κ)+ν·(∠x(k−κ+1)−2π) if ∠x(k−κ+1)>∠x(k−κ)

The above described delay can be implemented by cascaded delay elementsand associated sample storage. FIG. 5 illustrates such a delay subsystemwhich can be used as delay blocks 240, 270. As can be seen, delayelements 242A, 242B, 242C, 242D, 242E are cascaded and provide delaysand storage for input signal samples 244A, 244B, 244C, 244D, 244E.Switches 245A, 245B, 245C, 245D, 245E allow any one of the signalsamples 244A-244E to be switched so that it can be used. These samples244A-244E can be weighted accordingly by programmable weighting blocks246A, 246B, 246C, 246D, 246E. The weighted samples are then summed up byadder 248 to produce the delayed signal 249 to be used by the system.The switches 245A . . . 245E and the values in the weighting blocks 246A. . . 246E may be user/system controllable so that any combination ofweighted samples may be produced.

As an example, if τ·F₅=2.4 samples is required, then a value of 0.6 isused by the weighting block 246C and a value of 0.4 is used by theweighting block 245D. Then, by closing switches 245C and 245D then thesample x(k−2.4) is obtained.

The phase correction required for the feedback portion of the systemoutput signal is also dependent on the delayed signal x_(δ)(k). Thecomplex phase difference between the delayed signal x_(δ)(k) and thesystem output signal z(k), represented by γ, is due in part to the roundtrip delay and to other factors. The phase correction and delay can befound by executing the following process:

-   -   1. Reset the running-maximum register set P_(max)=0    -   2. Reset the phase correction: set ·γ=0    -   3. Preset the variable interpolating delay line (discussed above        as a delay subsystem):set δ=δ₀ (i.e. κ=κ₀ and ν=ν₀)    -   4. Perform a coarse search: select only interpolating delay line        integer sample-delay increments of δ (maintain ν=ν₀, increment κ        only): set the coarse and fine delay increments to Δκ=1 and        Δν=0.    -   5. Start with the first correlation window n0 . Time instance        k=0 signifies the commencement of the search.    -   6. “Integrate”: calculate the inner product        $P_{\delta} = {A_{MAC} \cdot {\sum\limits_{k = {nW}}^{{nW} + W - 1}{\exp\left( {j \cdot \left( {{\angle\quad{x_{\delta}(k)}} - {\angle\quad{z(k)}}} \right)} \right)}}}$        -    This is a complex multiply-and-accumulate (MAC) covering a            time window of W current samples. Note that the above            notation strictly implies that the correlation windows            n=0,1,2 . . . are adjoined, yet this is not an operational            requirement. A spacing between windows translates to a            lengthening of the search.    -   7. “Dump”: compare the complex magnitude |P_(δ)| with the value        P_(max), and if |P_(δ)|>P_(max) then        -   (a) update the maximum: set P_(max)=|P_(δ)|        -   (b) update the delay: retrieve the corresponding value of δ            and store it as δ_(max) and        -   c) update the angle: if fine searching store the angle            γ=∠P_(δ)    -   8. Proceed to the next window: set n=n+1, and increment δ by        Δκ+Δν    -   9. Repeat from Step 4, until the full integer (coarse) delay        range κ=κ₀ . . . K_(ν) of the interpolating delay line is        exhausted. ( K_(ν) is the maximum variable delay, κ₀ is the        starting coarse delay value for the search)    -   10. Store δ_(max) as δ₁.    -   11. Read the programmed fine search step size Δν.    -   12. Repeat Steps 1 and 4 to 8 for the fine search: employ        fractional-sample increments of Δν, covering the delays        δ₁−1≦δ≦δ₁+1, ensuring not to exceed 0≦δ≦K _(ν)    -   13. Store δ_(max) as δ₂.    -   14. Set and freeze the interpolating delay line delay to δ₂.    -   15. Apply the phase correction γ

Once the full course range is exhausted, the stored δ value δ_(max) isused a s a starting point for a fine search. With Δδ=0, the Δν isincremented fractionally so as to search the delays δ₁−1≦δ≦δ₁+1. Oncefound (using an analogous loop to that used for the coarse search), thephase correction γ is found along with the proper delay value. It shouldbe clear that γ=∠P_(δ).

Updating of the phase LUT is only to be accomplished after the phasesynchronization of the feedback signal. Freezing the phase LUT updatecan be done by setting μF=0 and μG=0.

After the phase correction is done, the magnitude of the feedback signalz(k) has to be adjusted as well. This adjustment is required tocompensate for any gain in the system output signal relative to theinput signal. Magnitude adjustment is accomplished by directlymultiplying the feedback signal z(k) by a factor A_(xx)(k). This isfound using the following formula:A _(xx)(k+1)=A _(xx)(k)+μ_(A)·(|x _(δ)(k)|·A _(xx)(k)·|z(k)|)For k _(A) ≦k≦k _(A+) W _(A)−1where

-   -   k_(A)=commencement time for the magnitude adjustments        (preferably as early as possible)    -   W_(A)=number of samples for which the adjustments are performed    -   μ_(A)=update step size and 0≦μ_(A)≦1 A_(xx)(k_(A)), μ_(A), and        W_(A) can be programmable.

Referring to FIG. 6, a circuit for implementing the magnitude adjustmentis illustrated. As can be seen, the magnitude 400 of the portion of thesystem output signal is multiplied with the factor A_(xx)(k) by way ofmultiplier 410. The result is to be used in calculating the desiredpredistortion modification. This result is also subtracted from themagnitude (|x_(δ)(k)|) of the delayed signal by way of adder 420. Theresult of the subtraction is multiplied by the update step size μ_(A) byway of multiplier 430 and added to the factor A_(xx)(k) by adder 440 toproduce the next value in the sequence for the factor. The delay element450 delays the resulting value until it is ready for use.

Once the delay, magnitude and phase adjustments have been made, newequations for the error signals can now be used:E _(m)(k)=A _(xx) ·|z(k)|−|x ₆₇ (k)|E _(φ)(k)=(γ+∠z(k)−∠x _(δ)(k))

These two error signals can therefore be used in place of the earlierones and these take into account the effects of the delay, magnitude,and phase differences between the system output signal and the inputsignal.

While the above embodiment utilizes LUT entries that are adjusted basedon the difference between the input signal and the system output signal,the system output signal alone may be used, if desired, in adjusting theLUT entries. This alternative would require the modulation of the systemoutput signal and the regeneration of a signal similar to the inputsignal. Successful recovery is possible if the amount of distortion isrestricted such that demodulation can occur without errors. Thisapproach is not desirable if the input signal is readily available to beused in determining the parameters of the adjustments. Anotheralternative involves measuring the distortion by measuring the amount ofout-of-band energy. Adjustments can then be made to minimize the amountof out-of-band energy and, consequently, minimize the distortion.

Regarding the amplifier subsystem 10, FIG. 7 illustrates one embodimentof the subsystem 10. In FIG. 7, the signal decomposer 20 of FIG. 1comprises a phasor fragmentation engine 20A along with phase modulationunits 60A, 60B. The fragmentation engine 20A receives the magnitude (M)and phase (φ) representing the predistorted signal. The phasorfragmentation engine 20A deconstructs a predetermined modulationwaveform (the predistorted signal) into signal components which are ofequal and constant magnitude. Further information regarding the phasorfragmentation engines may be found in the applicant's co-pendingapplication U.S. application Ser. No. 10/205,743 entitled COMPUTATIONALCIRCUITS AND METHODS FOR PROCESSING MODULATED SIGNALS HAVINGNON-CONSTANT ENVELOPES, which is hereby incorporated by reference. InFIG. 7, these signal components are denoted by angles α and β. Thesecomponents are each received by RF modulation and filtering blocks 60A,60B which process the components to produce RF modulated and filteredversions of the components. The signal component 70A is an RF signalwith phase a while signal component 70B is an RF signal with phase β.These components 70A, 70B are then amplified by amplifiers 90A, 90B. Theamplified components are then recombined using combiner 100. It shouldbe noted that the phase modulation, also known as carrier modulation,may also introduce some undesired amplitude modulation. Signaldecomposition methods other than the phasor fragmentation referred toabove may also be used by the signal decomposer 20.

Regarding the Chireix architecture amplifier subsystem 10, it has beenfound that, for higher amplification efficiencies, switch modeamplifiers are preferred for the amplifiers 90A, 90B. Such switch modeamplifiers, specifically Class D and Class F power amplifiers, providelow output impedances that allow higher amplification efficiencies. Aco-pending application filed on Oct. 16, 2002 and having U.S. Ser.No.10/272,725 entitled CHIREIX ARCHITECTURE USING LOW IMPEDANCEAMPLIFIERS provides further information on the desirable components andis hereby incorporated by reference. Such types of amplifiers are notrequired for the invention to function but they have been found toprovide performance at a desirable level.

It should further be noted that while those are only two parallelamplifiers 90A, 90B in FIG. 1 and FIG. 7, multiple parallel amplifiersmay be used as long as the decomposer 20 decomposes the predistortedsignal 130 into enough components so that each component is separatelyamplified and phase modulated in parallel with the other components.

It should also be noted that the predistortion subsystem 10 explainedabove does not linearize a power amplifier as is well-known in thefield. Instead, the predistortion subsystem linearizes a whole poweramplifier system—the output of the whole amplifier system is linearizedand not simply the output of a single amplifier. Also, unlike thelinearizing systems for power amplifiers that are currently known, theamplifier system discussed in this document compensates for distortionsthat mostly occur at mid signal amplitudes. Current single amplifierlinearization systems linearize distortions that occur at large signalamplitudes.

It should further be noted that the invention may be applied to anysignal processing system which decomposes a signal into components andrecombines them. It has been found that signal combiners (block 100 inFIG. 1) invariably cause distortions. These combiners use addition torecombine the components and improper signal addition, such as whenrecombining sinusoidal components, has been found to be one cause of thedistortions in the system output signal. In the above embodiment, thephasor fragmentation engine decomposes the incoming signal into vectorsand the improper addition of these vectors by the combiner 100 lead todistortions in the output signal.

While the above embodiment amplifies the input signal, albeit separatelyfor each component, this need not be the only signal processingaccomplished after the input signal is decomposed. Referring to FIG. 8,such a generalized system (which may be part of a larger signaltransmission system) is illustrated. The predistortion subsystem 120predistorts an incoming signal 30 and compensates for distortionsintroduced in the system output signal 110 by the improper or imperfectrecombining of the input signals components. These components areproduced by the signal decomposer 20 and are separately processed bysignal component processor blocks 75A, 75B. The processing executed bythe blocks 75A, 75B may take the form of amplification (as in theembodiment above), phase modulation, a combination of the two, or anyother signal processing which may be desired. As an example, each of thesignal components illustrated in FIG. 7 may be separately phasemodulated in addition to being amplified by amplifiers 90A-90B. Thephase modulation may be accomplished separately or be incorporated inthe signal decomposer or, as contemplated for the implementationillustrated in FIG. 7, incorporated into the modulation and filteringblocks 60A, 60B.

As can be seen in FIG. 8, the signal processing subsystem 10A receivesthe predistorted signal from the predistortion subsystem 120. Afterbeing received, the predistorted signal is decomposed by the signaldecomposer 20 into components. These components are then separatelyprocessed by the signal component processor blocks 75A, 75B and are thenrecombined by the recombiner 100.

A feedback signal processing block 400 receives a portion of the systemoutput signal 110 and processes this portion so it may be used by theadaptive predistortion subsystem 120. As an example, the feedback signalprocessing block 400 may contain the A/D converter 330 and theconversion unit 340 illustrated in FIG. 4. The magnitude adjustment andphase adjustment detailed above can also be implemented in this block400.

One advantage using the above invention is that it allows less stringenttolerances to be used for the system components. Previously, componentshad to be substantially matched so that signal processing could produceacceptable results. By using the above invention, less thansubstantially matched components may be used together. Errors due to amismatch may be measured and compensated for by the predistortionsubsystem.

Referring to FIG. 9, a detailed block diagram of a system incorporatingthe features illustrated in FIGS. 3, 4 and 8 is presented. As can beseen, the adaptive predistortion block 120 in FIG. 8 is comprised of theseparate magnitude delay 240 and phase delay 270 along with a magnitudepredistortion calculation block 260A. The inputs of these calculationblocks 230A, 260A are the delayed input signals from the delay blocks240, 270 and the adjusted feedback signals from the magnitude adjustmentblocks 410 and the phase adjustment block 420. After the magnitude andphase predistortion modification are calculated, then the magnitude LUTblock 220A and the phase LUT block 250A apply the predistortions. Itshould be noted that the magnitude LUT block 220A includes the magnitudeLUT 220 and the magnitude LUT update block 230 shown in FIG. 3.Similarly, the phase LUT block 250A incorporates the phase LUT 250 andthe phase LUT update block 260 illustrated in FIG. 3.

As can also be seen, the feedback signal processing block 400illustrated in FIG. 8 is comprised of the Cartesian to polar coordinateconversion block 340 which feeds the magnitude adjustment block 410 andthe phase adjustment block 420.

A person understanding this invention may now conceive of alternativestructures and embodiments or variations of the above all of which areintended to fall within the scope of the invention as defined in theclaims that follow.

1. A system for processing an input signal, the system comprising: anadaptive predistortion subsystem for receiving said input signal and forproducing a predistorted signal by applying a deliberate predistortionto said input signal; a signal processing subsystem receiving andprocessing said predistorted signal and producing a system outputsignal: a feedback signal processing subsystem for receiving andprocessing a feedback signal derived from said system output signal; anda delay subsystem for providing a delay to a replica of said inputsignal to produce a delayed signal, said delayed signal being used bysaid adaptive predistortion subsystem and said feedback processingsubsystem, wherein said adaptive predistortion subsystem distorts saidinput signal to compensate for distortions in said system output signal;said signal processing subsystem decomposes said predistorted signalinto separate components, each of said separate components beingprocessed separately; said processing subsystem combines said componentsafter processing to produce said system output signal; an output of saidfeedback processing subsystem being used by said adaptive predistortionsubsystem; said deliberate predistortion applied to said input signal bysaid adaptive predistortion subsystem to produce said predistortedsignal is adjusted based on said system output signal.
 2. A systemaccording to claim 1 wherein said signal processing subsystem comprises:a signal decomposer for decomposing said predistorted signal into atleast two components; at least two signal component processor blocks,each signal processor block receiving an output of said signaldecomposer and each signal processor block separately processes saidoutput received from said signal decomposer; and a combiner receiving aprocessed output from each of said at least two signal componentprocessor blocks, said combiner producing said system output signal fromsaid processed outputs of said at least two signal component processorblocks.
 3. A system according to claim 2 wherein at least one of said atleast two signal component processor blocks includes an amplifier.
 4. Asystem according to claim 3 wherein said amplifier is a non-linearamplifier.
 5. A system according to claim 1 wherein said system is partof a signal transmission system.
 6. A system according to claim 1wherein at least some of said distortions are due to said combiner.
 7. Asystem according to claim 3 wherein said amplifier is a switch modeamplifier.
 8. A system according to claim 3 wherein said amplifier has alow output impedance.
 9. A system according to claim 1 wherein saiddeliberate predistortion includes magnitude distortions which adjust amagnitude of said input signal.
 10. A system according to claim 1wherein said deliberate predistortion includes phase distortions whichadjust a phase of said input signal.
 11. A system according to claim 1wherein said deliberate predistortion is based on at least one entry ina lookup table.
 12. A method of processing an input signal to produce asystem output signal, the method comprising: a) receiving said inputsignal; b) applying a deliberate predistortion to said input signal toresult in a predistorted signal; c) decomposing said predistorted signalinto at least two component signals; d) combining said at least twocomponent signals to produce said system output signal; e) adjustingsaid deliberate predistortion based on said system output signal f)concurrent with steps b) -e), executing the following steps: f1)delaying a replica of said input signal; f2) diverting a divertedreplica of said system output signal to a feedback path; f3) processingsaid diverted replica to produce a corrected version of said divertedreplica; and f4) determining said deliberate predistortion using saidcorrected version and said replica of said input signal.
 13. A methodaccording to claim 12 wherein said system output signal is an RFmodulated version of said input signal.
 14. A method according to claim12 further including a processing step of separately processing each ofsaid at least two component signals prior to step d).
 15. A methodaccording to claim 14 wherein said processing step includes amplifyingat least one of said at least two component signals.
 16. A methodaccording to claim 14 wherein said processing step includes phasemodulating at least one of said at least two component signals.
 17. Amethod according to claim 12 wherein step a) further includes the stepof accessing an entry in a lookup table, said deliberate predistortionbeing based on said entry.
 18. A method according to claim 17 whereinsaid deliberate predistortion is based on an interpolation of entries insaid table.
 19. A system according to claim 1 wherein said predistortionsubsystem receives a replica of said system output signal.
 20. A systemaccording to claim 2 wherein said deliberate predistortion is dependenton differences between said input signal and said replica of said systemoutput signal.
 21. A system according to claim 11 wherein entries insaid lookup table are periodically updated based on characteristics of areplica of said system output signal.
 22. A system according to claim 11wherein said deliberate predistortion is based on an interpolation ofentries in said table.
 23. A system according to claim 1 wherein saidpredistortion subsystem includes: determining means for determining saiddeliberate predistortion; adjustment means for applying said deliberatepredistortion to said input signal; update means for periodicallyupdating said determining means based on said system output signal. 24.A system according to claim 23 wherein said adjustment means receivesparameters of said deliberate predistortion from said determining means.25. A method according to claim 12 further including the step of takinga difference between said replica of said input signal and said divertedreplica of said system output signal to determine said characteristicsof said system output signal.
 26. A method according to claim 17 furtherincluding the step of updating at least one entry in said table.
 27. Anadaptive predistortion subsystem for use with a signal processing systemwhich produces a system output signal, the predistortion subsystemcomprising: determining means for determining a deliberate predistortionto be applied to an input signal; adjustment means for applying saiddeliberate predistortion to said input signal; update means forperiodically updating said determining means based on characteristics ofsaid system output signal wherein said adaptive predistortion subsystemuses an output of a delay subsystem for delaying said input signal andan output of a feedback processing subsystem for processing a feedbacksignal derived from said system output signal to determine saiddeliberate predistortion.
 28. An adaptive predistortion subsystemaccording to claim 27 wherein said adjustment means receives parametersof said deliberate predistortion from said determining means.
 29. Anadaptive predistortion subsystem according to claim 27 wherein saiddetermining means comprises a lookup table having entries, said entriesbeing used to determine said deliberate predistortion.
 30. An adaptivepredistortion subsystem according to claim 29 wherein said determiningmeans further comprises interpolating means for determining values notfound in said lookup table.
 31. An adaptive predistortion subsystemaccording to claim 27 wherein said deliberate predistortion is dependenton differences between said input signal and said replica of said systemoutput signal.
 32. A system according to claim 1 wherein said delaysubsystem comprises: a plurality of delay elements; means for samplingsaid input signal; means for storing samples of said input signal; meansfor selecting selected samples of said input signal; and means forcombining said selected samples of said input signal.
 33. A systemaccording to claim 1 wherein said delay subsystem comprises separatesubsystems for separately delaying a magnitude and a phase of said inputsignal.
 34. A system according to claim 1 wherein said feedback signalprocessing subsystem comprises means for adjusting a phase of a replicaof said system output signal.
 35. A method according to claim 12 whereinsaid processing in step f3) comprises adjusting a phase of said divertedreplica based on said input signal.
 36. A method according to claim 12wherein said processing in step f3) comprises adjusting a magnitude ofsaid diverted replica based on said input signal.
 37. An adaptivepredistortion subsystem according to claim 27 wherein said delaysubsystem comprises: a plurality of delay elements; means for samplingsaid input signal; means for storing samples of said input signal; meansfor selecting selected samples of said input signal; and means forcombining said selected samples of said input signal.
 38. A method ofinitializing a phase correction to be applied to a feedback signal, saidfeedback signal to be used in determining a deliberate predistortion fora signal processing system, the method comprising: a) initiating acoarse delay search b) selecting a time window of W samples of saidfeedback signal and an input signal with a predetermined sample delayincrements of δ between samples c) calculating an inner product P_(δ) byperforming a complex multiply and accumulate process for the W samplesin the time window d) storing a maximum |P_(δ)| found e) repeating stepsc) and d) for subsequent time windows and incrementing δ by apredetermined amount for each time window f) repeating steps b)-e) for afine delay search using factional sample increments to cover apredetermined delay range, said delay range being centered on a maximumdelay increment δ_(max) found during said coarse delay search.
 39. Amethod according to claim 38 wherein said inner product process isdefined by$P_{\delta} = {A_{MAC} \cdot {\sum\limits_{k = {nW}}^{{nW} + W - 1}{\exp\left( {j \cdot \left( {{\angle\quad{x_{\delta}(k)}} - {\angle\quad{z(k)}}} \right)} \right)}}}$where ∠x_(δ)(k) is a phase of said input signal ∠z(k) is a phase of saidfeedback signal A_(MAC) is a constant n is an integer denoting a timewindow
 40. A method according to claim 38 wherein said phase correctionis a phase of said maximum P_(δ).
 41. A method according to claim 36wherein said magnitude is adjusted by multiplying said diverted replicaby A_(xx)(k) whereA _(xx)(k+1)=A _(xx)(k)+μ_(A)·(|x _(δ)(k)|−A _(xx)(k)·|z(k)|)For k _(A) ≦k≦k _(A+) W _(A)−1 where k_(A)=commencement time formagnitude adjustments W_(A)=number of samples for which adjustments areperformed μ_(A)=update step size and 0≦μ_(A)≦1 x_(δ)(k) is said inputsignal z(k) is said feedback signal.
 42. A method according to claim 35wherein said phase is adjusted by applying a phase correction to saiddiverted replica, said phase correction being initialized by a methodfor initializing a phase correction to be applied to a feedback signal,said feedback signal to be used in determining a deliberatepredistortion for a signal processing system, the method comprising: a)initiating a coarse delay search b) selecting a time window of W samplesof said feedback signal and an input signal with a predetermined sampledelay increments of δ between samples c) calculating an inner productP_(δ) by performing a complex multiply and accumulate process for the Wsamples in the time window d) storing a maximum |P_(δ)| found e)repeating steps c) and d) for subsequent time windows and incrementing δby a predetermined amount for each time window f) repeating steps b)-e)for a fine delay search using fractional sample increments to cover apredetermined delay range, said delay range being centered on a maximumdelay increment δ_(max) found during said coarse delay search.
 43. Amethod according to claim 12 wherein said deliberate predistortion is atleast partially based on said system output signal.
 44. A methodaccording to claim 43 wherein said deliberate predistortion isdetermined in an iterative manner during transmission of said systemoutput signal.
 45. A system according to claim 1 wherein saidpredistorted signal is adjusted based on said system output signal andsaid input signal.
 46. A method according to claim 12 wherein for stepe), said deliberate predistortion is adjusted based on said systemoutput signal and said input signal.
 47. A system according to claim 23wherein said update means periodically updates said determining meansbased on said system output signal and said input signal.
 48. A methodaccording to claim 38 wherein said inner product P_(δ) is based on aphase of said input signal and on a phase of a system output signal.